1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system therein, and in particular relates to an image forming apparatus for use in copying machines and facsimile machines, etc., for example.
2. Description of the Related Art
Recently, a so-called tandem-type image forming apparatus has been proposed as a multicolored or full-colored electrophotographic apparatus, in which plural photosensitive drums are arranged in a line. Each drum corresponds to a different color, and a toner image formed on each drum is sequentially overlaid on a recording member to form a color image thereon.
FIG. 7 is a schematic block diagram showing an example of a conventional full-colored tandem-type image forming apparatus employing an electrophotographic system therein.
The image forming apparatus comprises four image forming sections (image forming units) which are an image forming section 101M for forming a magenta image, an image forming section 101C for forming a cyan image, an image forming section 101Y for forming a yellow image, and an image forming section 101Bk for forming a black image. These four image forming sections are arranged in a line and spaced at predetermined intervals.
These image forming sections 101M, 101C, 101Y, and 101Bk are provided with photosensitive drums 102a, 102b, 102c, and 102d, respectively. Around the photosensitive drums 102a, 102b, 102c, and 102d, charging rollers 103a, 103b, 103c, and 103d, developing devices 104a, 104b, 104c, and 104d, transfer rollers 105a, 150b, 105c, and 105d, and drum cleaning devices 106a, 106b, 106c, and 106d are arranged, respectively. In upper portions between the charging rollers 103a, 103b, 103, and 103d and the developing devices 104a, 104b, 104c, and 104, exposure devices 107a, 107b, 107c, and 107d are arranged, respectively. In the developing devices 104a, 104b, 104c, and 104d, magenta toner, cyan toner, yellow toner, and black toner are stored, respectively.
In order to form an image on a recording member, each toner image formed on each photosensitive drum is transferred to the recording member which is carried on and conveyed by a conveying belt, thereby sequentially overlaying each toner image thereon. Then, using a fuser, the toner image is fixed on the recording member and the recording member is separated from the conveying belt and discharged from the apparatus.
By the way, when using the above-mentioned color image forming apparatus, not only full-colored printing, but also mono-colored (black) printing is frequently performed, and the rate of the latter case tends to increase recently.
However, in the above-mentioned conventional color image forming apparatus, even during the mono-colored (black) image forming, the four image forming sections (image forming units) 101M, 101C, 101Y, and 101BK are activated. Accordingly, the surfaces of the photosensitive drums 102a, 102b, and 102c of the image forming sections 101M, 101C, and 101Y which do not form images are mechanically ground just like during the full-colored printing due to the friction of the charging rollers 103a, 103b, and 103c, the developing devices 104a, 104b, and 104c, the drum cleaning devices 106a, 106b, and 106c, and a recording member P. That is, during the mono-colored printing, the losses of the surfaces of the photosensitive drums 102a, 102b, and 102c of the image forming sections 101M, 101C, and 101Y which do not form images are also caused by abrasion.
Also, in order to equalize the charge on the entire surface of the photosensitive drum when charging it by using the charging roller, in the voltage applied to the charging roller, a DC voltage is superposed on an AC voltage. In such a structure, due to the AC voltage, the surfaces of the photosensitive drums 102a, 102b, and 102c, which do not form images may be degraded, thereby reducing an endurance life thereof. This is mainly because the degradation of the photosensitive drum is caused by the electric discharge that occurs between the charging roller and the photosensitive drum due to the AC voltage.
Moreover, in recent color image forming apparatuses, a process cartridge may be used for improving usability, in which the charging device, the developing device, a toner container, etc., are integrated into the photosensitive drum. When mono-colored (black) printing is frequently performed in such an image forming apparatus, the degradation of the photosensitive drum may be advanced even when color toner is not consumed in the process cartridge. Thus, the life of the photosensitive drum can end before the color toner is fully consumed, thereby causing the remaining toner to be undersirably abandoned.
Accordingly, it is an object of the present invention to provide an image forming apparatus capable of increasing the life of an image carrier.